In complex modern electronic systems, such as computer and control systems, the provision of regulated power supply voltages to the circuits in the system is a necessity in order to ensure optimal system performance. A conventional technique, particularly useful for modular systems, is to provide multiple regulated power supplies. Such a modular approach allows for the addition of another functional module, such as an add-on board for a computer system, with only the addition of its own regulated power supply of the appropriate size. In this way, module additions may be made in virtually unlimited fashion, without concern that a single master power supply will be overloaded by the addition of an incremental module. Overall system reliability may also be enhanced according to this concept by providing n+1 regulated power supplies for n modules, thus providing a spare regulated power supply in the event of failure of a power supply failure or an overload condition.
A widely-used type of power supply controller is the pulse-width-modulated (PWM) controller. As is well known, PWM power supply controllers provide, at their outputs, a series of pulses of varying pulse width according to the difference between a feedback signal corresponding to the actual power supply voltage (as applied to the load) and the reference voltage at which the voltage is to be regulated (i.e., the commanded level). Two modes of PWM power supply control are conventionally used, namely voltage mode and current mode.
Voltage mode PWM power supply control is accomplished by comparing the output power supply voltage to a reference level, generating an error signal corresponding to the difference therebetween. The error signal is then compared against an oscillating sawtooth signal from a free-running R-C oscillator, resulting in a PWM square wave which is applied to the output. Examples of voltage mode PWM power supply controllers include the L296, L4960, L4962 and L4964 switching regulators manufactured and sold by SGS-Thomson Microelectronics, Inc. Current mode PWM power supply control is accomplished by sensing the current supplied by the power supply controller to the load (generally the primary of a transformer), comparing it against the commanded level, and toggling a series of latches according to the comparison at a frequency established by an R-C oscillator. Examples of current mode PWM power supply controllers include the UC1842/2842/3842 series of controllers manufactured and sold by SGS-Thomson Microelectronics, Inc., and the UC3825 controller manufactured and sold by Unitrode. In each of the voltage and current mode cases, the R-C time constant of the oscillator is generally defined by an external R-C network connected to the appropriate terminals of the controller. While both modes of PWM power supply control are useful, current mode control is preferable for many systems, as the output current from each of the multiple power supply controllers in the system can be easily shared.
In the case of a system having multiple current mode PWM power supply controllers, synchronization of the power supply controllers is highly desirable. Since the controller outputs are pulses, a significant amount of electromagnetic interference is generated from output switching transients. It is well known that if the multiple power supply controllers are synchronized with one another, the frequencies of the switching noise are limited so that the noise can be readily filtered.
Multiple PWM power supply controllers can be synchronized with one another by synchronizing, in frequency and phase, the oscillators of each of the controllers As noted above, voltage mode PWM controllers each include an oscillator for generating the sawtooth input to the comparator, and current mode PWM controllers each generally include an oscillator for setting the frequency of the PWM output. Prior synchronization techniques include the definition of a master signal (generated either by one of the oscillators or by an external source), and running all, or all but one, of the oscillators in a slaved fashion thereto.
An externally generated signal is one obvious form of a master signal which, when applied to each of the PWM controllers, can control the operation of each of the oscillators therein. For example, an externally generated sawtooth signal may be directly applied to the comparators of each of the multiple voltage mode PWM controllers. However, since in many voltage mode PWM controllers (e.g., the L296 and L4964 switching regulators) the internal oscillators must be disabled when run by an external signal, loss of the external signal renders the controllers inoperable. In addition, complex additional circuitry is required in such a scheme. In some PWM controllers (e.g., the L4960 and L4962 switching regulators), the internal oscillator cannot be disabled when an external signal is applied, in which case prior schemes have used an external synchronization signal to maintain synchronization. The external sync signal must be carefully designed, however, so that multiple pulses are not generated (as occurs when the external frequency is too low) nor is the internal oscillator defeated (as occurs when the external frequency is too high).
According to another prior synchronization technique, the oscillators of multiple PWM controllers are coupled together, with one of the controllers serving as the master and the others as slaves. A typical method of setting up master and slave controllers is to operate one controller (i.e., the master) at a higher frequency than that at which the others (slaves). Connection of the synchronization terminals of the master and slave controllers to one another will allow the master controller to dominate the slave controllers and control the operation of all of the controllers in the system.
This master and slave configuration limits the design and manufacture of multiple power supply systems in certain ways, however. First, since the master and slave controllers are preferably of different types, (i.e., different operating specifications), both types of controllers must be maintained in manufacturing inventory, complicating production of the system. Secondly, it is essential in such arrangements that the oscillator frequencies of the master and slave controllers do not overlap, as otherwise the slave oscillators can generate extra pulses. This requires the timing components in the system to be of twice the precision otherwise necessary, and also requires that the operation of one or both of the controllers be held back from its full performance specifications to account for frequency tolerance limits of the slave controller (on the order of .+-.10%), each of these effects resulting in the use of more costly components than otherwise necessary for given system specifications. In addition, these prior master/slave arrangements have limited fan-out and thus the number of slave controllers allowed is limited. These arrangements can also be quite susceptible to false triggering caused by ground noise, which adds jitter to the regulated power supply output.
It should also be noted that the master/slave arrangement is also vulnerable to the failure of the master controller, in which case the synchronization of the slave controllers will be lost (or, alternatively, the slave controllers will be inoperable). Accordingly, the desired redundancy noted hereinabove is not fully achievable using this arrangement.
It is therefore an object of this invention to provide a power supply controller which provides for automatic synchronization in a multiple power supply scheme.
It is a further object of this invention to provide a system incorporating multiple ones of such controllers.
It is a further object of this invention to provide such a system which synchronizes all controllers to the fastest oscillator presently in the system.
It is a further object of this invention to provide such a system where all of the controllers in the system are of the same type.
It is a further object of this invention to provide such a system which provides a single-ended synchronization pulse to all controllers.
It is a further object of this invention to provide such a system which provides a differential synchronization signal for improved noise immunity.
Other objects and advantages of this invention will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.